In the oil and gas industry, modeling of the subsurface is typically utilized for visualization and to assist with analyzing the subsurface volume for potential locations for hydrocarbon resources. Accordingly, various methods exist for estimating the geophysical properties of the subsurface volume (e.g., information in the model domain) by analyzing the measurements from measurement equipment (e.g., information in the data domain). The measurements include some information of the geophysical properties that may be utilized to generate the subsurface model.
The interpretation of the subsurface volume may be complicated by the rock properties within the subsurface. In particular, interest in the anisotropic strength characteristics exhibited by fine-grained rock formations has resulted primarily from a desire to understand wellbore failure mechanisms associated with drilling through shales. Although elastic anisotropy in shales can impact wellbore failure through the redistribution of borehole stresses, analytical studies have shown that strength anisotropy has a more significant control. See e.g., Gazaniol, D., T. Forsans, M. J. F. Boisson and J.-M. Piau. 1995. Wellbore failure mechanisms in shales: Prediction and prevention. J. Pet. Tech. July 1995: 589-595.
This effect is particularly pronounced for extended reach drilling (ERD) where the borehole trajectory can achieve near-parallelism with weak bedding planes, as shown in FIG. 1. If the attack angle is low (e.g., approximately 70 to 90° borehole inclination through a formation with approximately (≈) 0° dip angle) interaction between the borehole hoop stress (σθ in FIG. 1) and the anisotropic formation can result in oblique loading of the relatively weak bedding laminations leading to premature shear failure. This failure mechanism is often attributed to the invasion of drilling fluids into micro-fractures along bedding planes, which is not necessarily the cause. See, e.g., Ottesen, S. 2010. Wellbore stability in fractured rock. In Proceedings of the IADC/SPE Drilling Conference, New Orleans, 2-4 Feb. 2010. Depending on the relative magnitudes of anisotropic rock strength and borehole stress concentration, breakouts may occur at unexpected positions around the borehole different from the more typical elliptical elongation conventionally found in isotropic rock. See, e.g., Willson, S. M., S. T. Edwards, A. Crook, A. Bere, D. Moos, P. Peska and N. Last. 2007. Assuring stability in extended-reach wells—Analyses, practices and mitigations. In Proceedings of the IADC/SPE Drilling Conference, Amsterdam, 20-22 Feb. 2007.
For optimally orientated positions around the borehole circumference, failure may occur via oblique loading of the planes of weakness (intra-laminar failure) before failure can occur within the bulk material cutting through the planes of weakness (trans-laminar failure), necessitating higher mud weights to reduce stress concentration on weak bedding planes, or else limiting wellbore inclination.
As the recovery of natural resources, such as hydrocarbons rely, in part, on a subsurface model, a need exists to enhance subsurface models of one or more geophysical properties. In particular, a technique for predicting the degree of shear strength anisotropy encountered in fine-grained rock formations (including shales) without recourse to costly and time consuming laboratory measurements is needed. The technique may preferably be suited for utilizing geophysical wireline logs as input data to the predictive technique for evaluating strength anisotropy. The technique calculates a lithology-dependent anisotropic failure criterion that can subsequently be input into analytical or computational methods for optimizing mud weight design in ERD wells. See, e.g., Ottesen, S., R. H. Zheng and R. C. McCann. 1999. Borehole stability assessment using quantitative risk analysis. In Proceedings of the IADC/SPE Drilling Conference, Amsterdam, 9-11 Mar. 1999 and Pande, G. N., G. Beer and J. R. Williams. 1990. Numerical Methods In Rock Mechanics. John Wiley & Sons, NY.